1. Field of the Invention
This invention relates to a microphone apparatus for a video camera which is equipped with an optical zoom lens and microphones to provide video and audio signals to a video tape recorder. More particularly, this invention relates to a microphone apparatus the directivity of which can be varied in accordance with the control of the zoom lens to suitably match images and sounds.
2. Description of the Prior Art
Microphone apparatus for producing acoustic zoom effects on the basis of monaural sound recording in which the directivity or both the directivity and the sensitivity of a microphone are varied have been proposed (for example, Y. Ishigaki et al.: "Zoom Microphone" presented at the 67th Convention of the Audio Engineering Society (Oct. 31 to Nov. 3, 1980), pre-print 1713 (A-7), and U.S. Pat. No. 3,837,736).
In such microphone apparatus, it is impossible to harmonize the working angle of the microphone with the zooming magnification of the zoom lens, resulting in that reproduced sounds are not adequately matched with reproduced images. This imbalance between the working angle and the zooming magnification will be described below by way of illustrating a video camera having a zoom lens of ten magnification and a microphone having a variable directivity (from omnidirectional to second order pressure gradient unidirectional through unidirectional). The viewing angle of the zoom lens of ten magnification in the telescopic state is very narrow (about 4 degrees), while it is about 40 degrees in the wide viewing angle state. The working angle at half maximum of the variable-directivity microphone is 360 degrees in the omnidirectional state, 180 degrees in the unidirectional state, and 100 degrees in the second order pressure gradient unidirectional state (i.e., the telescopic state). Namely, the working angle of the variable-directivity microphone is very wide in comparison with the viewing angle of the zoom lens. When defining an acoustic zooming magnification of a microphone as the ratio of the distance factor in the wide viewing angle mode to that in the telescopic mode, the acoustic zooming magnification of the above-mentioned variable-directivity microphone is about 2.7, i.e., the acoustic zooming magnification of the microphone is very much smaller than the zooming magnification of the optical zoom lens. The distance factor means a distance from a sound source to a directional microphone which is positioned so as to produce noise and echo signals equivalent to those obtained in an omnidirectional microphone separated from a sound source by a unit distance. The distance factors of an omnidirectional microphone, a unidirectional microphone and a second order pressure-gradient microphone are 1, about 1.7 and about 2.7, respectively (see, for example, "An anthology of articles on microphones" from the pages of the Journal of the Audio Engineering Society vol 1-vol. 27 (1953-1979)", p.62). To eliminate the above-mentioned imbalance, the development of a superdirectional microphone having an extremely sharp directivity is essential. With the present technology, however, it is impossible to realize a microphone having such a directivity and the capability of being built into a video camera which must be compactly constructed.
A microphone apparatus which can produce acoustic zoom effects on the basis of two-channel stereo sound recording is known. An apparatus of this type is disclosed as a stereo sound processor for television broadcasting, in U.S. Pat. No. 4,594,610. This processor is designed to use two microphones to conduct a stereo recording. The two microphones are disposed on the right and left of a stage, separately from a video camera, to generate microphone signals L and R from which a sum signal (L+R) and a difference signal (L-R) are produced. In order to produce monophonic sounds in the telescopic mode and stereophonic sounds in the wide viewing mode, the mixing ratio of the sum signal (L+R) and the difference signal (L-R) is controlled in accordance with the control of the zoom lens of the video camera, so that the monophonic sum signal (L+R) is greater than the difference signal (L-R) in the telescopic mode, and, to the contrary, the difference signal (L-R) is greater than the monophonic sum signal (L+R) in the wide viewing angle mode. This apparatus can solve a problem in a two-channel stereo recording and reproducing system in which the sum signal (L+R) and the difference signal (L-R) are delivered without any processing, which problem is that, When images and sounds are recorded simultaneously by panning the video camera to the left and zooming in, the zoomed subject is displayed largely in the center on the television image, but the sound is delivered only from the left loudspeaker.
In such an apparatus, since the microphone is not installed on the video camera, the pan operation of the video camera is performed regardless of the direction of the principal axis of the directivity of the microphone. When the sound source is positioned in the panned direction, therefore no problem is caused. But, when the sound source is not in the panned direction, the displayed image and the reproduced sound fail to coincide with each other. Even if the apparatus is installed on the video camera and a microphone having any directivity is employed, as far as the processing is executed on the basis of the two signals L and R, it is difficult to simultaneously satisfy both the quality (such as the localization, spaciousness and perspective) of stereo sounds required in the wide viewing angle mode and the quality (such as clearness) of monophonic sounds required in the telescopic mode, resulting in that either quality must be ignored.
A conventional microphone apparatus, for example, disclosed in the above-mentioned three references is provided with a signal means for generating a signal corresponding to the zooming magnification of the zoom lens. This signal means comprises a potentiometer the resistance of which can be changed by sliding a contactor which is driven in accordance with the movement of the zoom lens, through a means mechanically interconnecting the contactor and the zoom lens.
A conventional microphone apparatus provided with such a signal means has a problem in that mechanical vibrations propagate through the casing or the space to the microphone to generate noise signals. Since the resistance of the potentiometer must be high in view of the power consumption, the level of the noise electromagnetically induced in the potentiometer is so high that the noise is mixed into the signals of the microphone. In the potentiometer, moreover, noises are generated when the contactor of the potentiometer is slid, to be mixed into the signals of the microphone. Besides, a potentiometer, which is a slide-type variable resistor, has problems in reliability and reproducibility, resulting in that a potentiometer is not suitable to be used as a part which is frequently operated.